OAXACA, Mexico – Six people are dead and more than 100 injured during a teachers union protest in Oaxaca, Mexico over the weekend, the latest in a years long feud between the union and government.

Members of the National Coordinator of Education Workers union and their supporters hurled stones and Molotov cocktails, burned vehicles, and allegedly fired at police in Nochixlan and other locations in the southern state of Oaxaca on Sunday, the Associated Press reports.

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Government officials sent in police and federal agents to quell the uprising, resulting in six civilians killed, as well as 41 federal agents, 14 state police and 53 civilians injured in the violent clash. The Mexican government issued a statement alleging the initial police response included unarmed officers, but authorities later acknowledged police fired on protesters after three federal police were injured by gunfire.

“The attacks with guns came from people outside the blockades who fired on the population and federal police,” according to a government statement cited by the AP.

The news service recorded video footage of at least one officer firing at protestors.

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“The police obligation is to protect the population,” federal police chief Enrique Galindo said.

The violent weekend clash follows a week of unionized teachers blocking streets, malls, train tracks, and highway to the tourist town of Oaxaca, where they’ve also set up an encampment in the city square.

The revolt follows a tense relationship between the government and union that’s brewed for years. The disagreement centers on education reforms instituted three years ago that require mandatory teacher testing. Union officials also face corruption and money laundering charges stemming from allegations they funneled cash from teachers to fund protests and pad their pay while the union controlled payroll between 2013 and 2015, according to the news service.

“The protest on Sunday near the town of Nochixtlan, about 50 miles (80 km) northwest of Oaxaca City was the latest of several in recent days that saw protesters blocking other highways with burning tires,” Reuters reports.

“The unrest escalated after police arrested the leader of the local teachers’ union during the previous weekend. Ruben Nunez, head of one of the most combative factions of Mexico’s CNTE union, was detained on suspicion of corruption.”

Nunez is the secretary general of the union’s Section 22, the center of the resistance to reforms, according to the news site.

Two of the civilians killed over the weekend had ties to the CNTE union, but many involved in the mayhem were masked youngsters with a penchant for violence, officials said.

“These kinds of radicalized protests generate violence,” Galindo said.

According to the AP:

Ten years ago, the teachers started a six-month takeover of Oaxaca that didn’t end until police stormed the barricades.